Glasses and porcelains have been used as an insulation for a metal member in bushing applications for capacitors and pole-type power transformers, and as vacuum interrupter enclosures, lightning arrester housings, electric line insulators, and in many specialty applications as an insulating material. The insulating material-metal conductor seal is accomplished in a variety of ways; usually by preglassing, the metal component with, for example, borax glasses or SiO.sub.2 --PbO--K.sub.2 O or SiO.sub.2 --Li.sub.2 O--ZnO--B.sub.2 O.sub.3 systems, or by using an intermediate metallic sheath member which slides over and contacts the metal conductor. The sheath is easily bonded to the glass or porcelain and is also easily bonded to the metal but is an expensive component of the system. Direct bonding of porcelain to conductor is not practical because of the low reactivity between porcelain and normal conductor metals. Glass embedment is usually impractical because of thermal expansion mismatches between insulator glasses and economical conductors. However, adequate bonding has been accomplished in both cases with nickel-iron alloy intermediate sheath members.
With the development of the glass-ceramic process, a whole new avenue of material applications emerged, whereby glass technology could be employed in the initial forming of an article, such as an insulated bushing, and then a subsequent, special heat treatment would convert the glass article to a crystalline oxide material dispersed in a glass matrix, the composite normally having improved properties. The ability to regulate the crystallizing phase by composition and heat treatment permits the adjustment of physical properties, such as coefficient of thermal expansion and hardness, to suit the application.
WHILE GLASS-CERAMIC INSULATING COMPOSITIONS CAN BE ADJUSTED TO ADEQUATELY MATCH THE EXPANSION CHARACTERISTICS OF ECONOMICAL CONDUCTORS, GENERALLY THE BOND STRENGTH IN GLASS-CERAMIC EMBEDMENTS AND COATINGS IS QUITE POOR WHEN COMPARED TO GLASS ENAMEL BONDING. The strength of such embedments comes from the mechanical interlocking of the metal part in the rigid glass-ceramic, and only in small degree from glass-ceramic to metal interaction.
Glass-ceramic compositions are well known in the art, and are taught by Chen in U.S. Pat No. 3,006,775 and McMillan et al in U.S. Pat. No. 3,379,542 as an insulating material, where high strength must be coupled with good electrical insulating properties. The bonding of such compositions to pre-glassed metal components is taught by McMillan et al in U.S. Pat. No. 3,220,815. However, new ways of improving adherence and hardness in these systems, while still maintaining thermal expansion matching to suitable metal conductors are desirable.